Sullivan invented the treatment of Obstructive Sleep Apnea (OSA) with nasal Continuous Positive Airway Pressure (CPAP). (See U.S. Pat. No. 4,944,310.) Diagnosis of OSA typically requires two nights in a sleep clinic. During a first night, the patient is monitored to see whether the patient has OSA. During the second night, a range of nasal mask pressures are tested to determine an appropriate pressure setting for a CPAP device to keep the patient's airway open. Once a pressure setting is determined, the patient is prescribed a CPAP device set to that pressure for subsequent home treatment. Because of limited places in sleep clinics, a patient can wait up to two years before he has the opportunity to be diagnosed. More recently, automatic devices have been developed which can diagnose and treat patients in their own homes, reducing the delay. Some automatic devices also can increase and decrease the treatment pressure during the night in accordance with patient need.
U.S. Pat. No. 5,199,424 (Sullivan and Lynch) describes a device for monitoring breathing during sleep and control of CPAP treatment that is patient controlled. In particular, the patent describes a CPAP apparatus including a controllable variable-pressure air source; a nose piece for sealed air communication with a patient's respiratory system; an air communication line from the air source to the nose piece; a sound transducer adapted to be in sound communication with the patient's respiratory system; and a feedback system controlling the output pressure of the air source in response to an output from the transducer so as to increase the pressure of the air source, in response to detection of sound indicative of snoring, in accordance with a predefined procedure. The sound transducer, in its most general form, comprises a pressure transducer which, in addition to detecting snoring sounds, can detect other respiratory parameters such as the rate of breathing, inhaled air flow volume, and inhaled air flow rate. The output air pressure of the air source is increased in response to one or more of these parameters in accordance with a pre-defined procedure.
U.S. Pat. No. 5,134,995 (Gruenke et al.) is said to describe an apparatus and method for facilitating the respiration of a patient for treating mixed and obstructive sleep apnea and certain cardiovascular conditions, among others, by increasing nasal air pressure delivered to the patient's respiratory passages just prior to inhalation and by subsequently decreasing the pressure to ease exhalation effort. The preferred apparatus includes a patient-coupled air delivery device for pressurizing the patient's nasal passages at a controllable pressure, and a controller coupled with the delivery device having a pressure transducer for monitoring the nasal pressure and a micro-controller for selectively controlling the nasal pressure. In operation, the controller determines a point in the patient's breathing cycle just prior to inhalation and initiates an increase in nasal pressure at that point in order to stimulate normal inhalation, and subsequently lowers the nasal pressure to ease exhalation efforts.
U.S. Pat. No. 5,203,343 (Axe et al.) is said to describe a method and device for controlling sleep disordered breathing utilizing variable pressure. A compressor supplies air at a relatively low pressure to the user's air passages while the user is asleep. A pressure transducer monitors the pressure and converts the pressure to an electrical signal. The electrical signal is filtered and compared with the characteristics of waveforms that exist during snoring. If the envelope of the waveform exceeds an average threshold value in duration and in area, then a microprocessor treats the envelope as possibly being associated with a snore. If a selected number of envelopes of this nature occur within a selected time period, then the microprocessor considers snoring to exist and increases the pressure of the compressor. If snoring is not detected within a certain time period, then the microprocessor lowers the level gradually.
U.S. Pat. No. 5,335,654 (Rapoport) is said to describe, in the treatment of obstructive sleep apnea, a CPAP flow generator employed to direct air to a nasal mask fitted to a patient. The airflow from the generator is monitored, and the flow and/or pressure is increased when the waveform of the airflow exhibits characteristics corresponding to flow limitation. The generator may be controlled to repetitively test for waveform variations, in order to adjust the CPAP flow to the minimum level that does not produce flow limitation.
U.S. Pat. No. 5,704,345 (Berthon-Jones) describes a method and apparatus for detection of apnea and obstruction of the airway in the respiratory system. Methods and apparatus for determining the occurrence of an apnea, patency and/or partial obstruction of the airway are disclosed. Respiratory air flow from a patient is measured to provide an air flow signal. The determination of an apnea is performed by calculating the variance of the air flow signal over a moving time window and comparing the variance with a threshold value. One determination of partial obstruction of the airway is performed by detecting the inspiratory part of the air flow signal, scaling it to unity duration and area, and calculating an index value of the amplitude of the scaled signal over a mid-portion. Alternatively, the index value is a measure of the flatness of the air flow signal over the mid-portion. One determination of patency of the airway is performed by applying an oscillatory pressure waveform of known frequency to a patient's airway, calculating the magnitude of the component of the air flow signal at the known frequency induced by the oscillatory pressure waveform, and comparing the calculated magnitude with a threshold value. Alternatively, the air flow signal is analyzed to detect the presence of a component due to cardiogenic activity.
U.S. Pat. No. 6,367,474 (Berthon-Jones and Farrugia) describes CPAP treatment apparatus having a controllable flow generator operable to produce breathable air to a patient at a treatment pressure elevated above atmosphere by a delivery tube coupled to a mask having a connection with a patient's airway. A sensor generates a signal representative of patient respiratory flow that is provided to a controller. The controller is operable to determine the occurrence of an apnea from a reduction in respiratory airflow below a threshold and, if an apnea has occurred, to determine the duration of the apnea and to cause the flow generator to increase the treatment pressure by an amount which is an increasing function of the duration of the apnea, and a decreasing function of the treatment pressure immediately before the apnea.
In general, these types of techniques of control of the administration of CPAP treatment can be regarded as “micro-control” algorithms. That is, they monitor the condition of the patient at any given moment. A variety of techniques for monitoring the patient's condition may be used, including detecting flattening of the inspiratory flow-time curve, detecting a reduction in patient ventilation, and detecting snoring. If there is an indication of sleep disordered breathing, the response is to increase the treatment pressure. Absent the indication of sleep disordered breathing, treatment pressure may be decreased. A question of interest is how effective such algorithms are in treating OSA.
The Apnea-Hypopnea Index (AHI) provides a measure of the number of apneas and hypopneas which a patient experiences during sleep. The AHI is sometimes used to assist in diagnosis of OSA. The AHI may also be used as an indication of the effectiveness of the nasal CPAP treatment. Hence, at least one automatic CPAP device, the AUTOSET T™, manufactured by ResMed Limited, reports the AHI, among other things, after a night's treatment. A problem with such an approach is that if the AHI indicates that therapy has been ineffective, the device may not be able to respond to such a result without being adjusted by a technician or clinician.